Thus the saying "a prosecutor can get a ham sandwich incited."
That's certainly part of it, though the "ham sandwich" saying is really more about the fact that, since the prosecutor has effective control over the entire grand jury proceeding (which witnesses they hear from, what evidence they see, etc.), a prosecutor can pretty much get a grand jury to do he wants it to do. If a prosecutor wants an indictment, he'll get one (usually). If a prosecutor doesn't want an indictment, the grand jury will return no true bill (usually). Going through the grand jury evidence in the Wilson case, it is pretty clear that the latter is what happened - the prosecutor did not believe there was a case against Wilson (because there was not a case against Wilson), but was essentially forced by public pressure to present the case to the grand jury, so he presented a case that he knew couldn't possibly result in an indictment.